High strength seamless steel pipe and its manufacturing method

ABSTRACT

Anon-heat treated seamless steel pipe comprising, by weight, C: 0.10 to 0.25%, Si: 0.05 to 1.00%, Mn: 0.50 to 2.50%, P: not more than 0.03%, S: not more than 0.05%, Cr: 0.40 to 1.50%, Mo: 0.05 to 1.50%, V: 0.02 to 0.30%, Al: 0.003 to 0.10%, B: 0.0003 to 0.01%, and N: 0.001 to 0.02%, in which the balance comprises Fe and impurities. According to this non-heat treated seamless steel pipe, a ferrite area ratio is not more than 10%, and a carbon equivalent Ceq. (%) defined by the following equation is 0.45 to 0.85%. 
 
Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a seamless steel pipe for machine structural use and the like, and more particularly, it relates to a non-heat treated seamless steel pipe which can be used as being made into a hot-manufactured pipe, and has high strength, high toughness and excellent weldability.

2. Description of the Background Art

Conventionally, a seamless steel pipe for usage requiring high strength and high toughness has been manufactured as follows. That is, the seamless steel pipe is manufactured through a step of piercing a billet and a step of rolling in a hot process. Then, the steel pipe is quenched and tempered, whereby predetermined levels of strength and toughness are provided in a product.

According to the manufacturing method of the seamless steel pipe, since heat treatment is needed after the pipe is manufactured, its cost is increased and it takes time until the product is delivered. In order to solve the above problem, there is an increasing demand for a non-heat treated seamless steel pipe having high strength and high toughness.

Conventionally, the non-heat treated seamless steel pipe used in a hydraulic cylinder and the like is mostly 540 MPa steel. This “540 MPa steel” means steel having strength in which tensile strength is not less than 540 MPa and yield stress is not less than 390 MPa. When the steel pipe used in the hydraulic cylinder is highly pressurized and reduced in weight, high strength heat treated steel which went through quenching and tempering steps is used. In order to provide a high strength non-heat treated seamless steel pipe, some attempts have been made, that is, carbon loadings are increased or a large amount of expensive alloy element is applied, for example. However, weldability is lowered and a cost is increased in the above attempts.

The above problems will be described in more detail with reference to the conventional related art.

Non-heat treated seamless steel pipes having high strength and high toughness are disclosed in Japanese Unexamined Patent Publication No. 03-162524, Japanese Unexamined Patent Publication No. 05-202447, Japanese Unexamined Patent Publication No. 09-25541, Japanese Unexamined Patent Publication No. 10-130783, Japanese Unexamined Patent Publication No. 10-204571, Japanese Unexamined Patent Publication No. 10-324946, Japanese Unexamined Patent Publication No. 11-36017, Japanese Unexamined Patent Publication No. 2000-328192, Japanese Unexamined Patent Publication No. 2001-323338, Japanese Unexamined Patent Publication No. 2001-247931, and Japanese Unexamined Patent Publication No. 2001-262275, for example.

The Japanese Unexamined Patent Publication No. 03-162524 discloses a method of manufacturing a high tensile seamless steel pipe having excellent low-temperature toughness. More specifically, a material is heated up to 1150 to 1300° C. and precooling process by forcible cooling and reheating process are performed while the pipe is produced and then final forcible cooling is performed for it to miniaturize crystal grains in order to increase the toughness and the strength. According to this method, a precooling apparatus is needed when the pipe is produced and a forcible cooling apparatus is needed after the pipe is produced, which is a problem.

In each of the Japanese Unexamined Patent Publication No. 05-202447, the Japanese Unexamined Patent Publication No. 09-25541, the Japanese Unexamined Patent Publication No. 10-130783, the Japanese Unexamined Patent Publication No. 10-205671, the Japanese Unexamined Patent Publication No. 10-324946, the Japanese Unexamined Patent Publication No. 11-36017, and the Japanese Unexamined Patent Publication No. 2000-328192, component adjustment and a method of manufacturing a pipe in a hot working process in order to provide the non-heat treated steel pipe having the high strength and the high toughness are disclosed. In addition, it is in common among the above Japanese Unexamined Patent Publications that carbon (C) is added 0.2% or more to design a medium carbonaceous component. Thus, since carbon is added 0.2% or more, the toughness is not sufficient with respect to the strength. Especially, at a welding part, the toughness is lowered because of quench hardening or a weld crack is generated.

The Japanese Unexamined Patent Publication No. 2001-323338 discloses a method of implementing hot workability, machinability and toughness in steel having a broad range of carbon loadings. However, since vanadium (V) is not added in a steel pipe disclosed in this document, sufficient strength cannot be provided.

The Japanese Unexamined Patent Publication No. 2001-247931 and the Japanese Unexamined Patent Publication No. 2001-262275 disclose technique for providing the strength and the toughness and technique for providing the hot workability by restricting a manufacturing temperature in a hot working process so as to control a metal structure in steel having a broad range of carbon contents. However, in order to implement low-temperature manufacturing disclosed in the above documents, it is necessary to convert a facility because a motor power is insufficient in the conventional facility. Furthermore, a reheating furnace and the like is additionally needed in order to reheat a pipe after cooling it once to manufacture the pipe.

Still further, according to the Japanese Unexamined Patent Publication No. 2001-247931, although a broad range of carbon contents is defined in claims, carbon is added 0.2% or more in the description in the embodiment.

According to a method disclosed in the Japanese Unexamined Patent Publication No. 2001-262275, although a pipe is cooled down once and reheated again to 900° C. after the pipe is manufactured, V, Ti, Nb and the like are precipitated when the temperature is lowered once, and these precipitations grow in the subsequent reheating process. As a result, the precipitations become large, so that the toughness is lowered and sufficient strength is not provided.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a non-heat treated seamless steel pipe having both high strength and high toughness and having preferable weldability. Especially, it is an object of the present invention to provide a high strength non-heat treated seamless steel pipe in which the tensile strength is not less than 640 Mpa and the yield stress is not less than 490 Mpa.

It is another object of the present invention to provide a method of manufacturing the non-heat treated seamless steel pipe having the above characteristics.

The inventor of the present invention found that the following things were effective in order to attain the above objects.

(1) A carbon content was lowered. Then, in order to compensate strength because the carbon content was lowered, manganese (Mn), chromium (Cr) and vanadium (V) were added together. Thus, high strength and preferable toughness were provided in the pipe including the welding part.

(2) A ferrite area ratio was lowered to a predetermined level or less. Thus, predetermined strength was provided. The “ferrite area ratio” was measured as an area ratio of a ferrite crystal existing in the steel pipe to its visual field, by means such as an optical microscope.

(3) The carbon content was lowered and then a carbon equivalent (Ceq.) is adjusted in a predetermined range.

The standard is that a metal structure of the non-heat treated steel comprises ferrite-pearlite steel. However, when it was highly carbonized in order to meet the demand for high strength, it was found that the toughness was lowered. Then, the inventor of the present invention lowers the carbon content and added Mn, Cr and V together in order to compensate the strength. Thus provided metal structure of the non-heat treated steel becomes a bainite-based structure, so that the high strength and the high toughness can be secured. Not only the term “bainite-based constitution” includes a structure in which bainite exists by 100%, but also it includes a mixture consisting of bainite and ferrite in which volume % of ferrite is 50 or less.

The essential features of the present invention are as follows, that is,

-   (1) the carbon content is controlled to be 0.25% or less, -   (2) manganese (Mn), chromium (Cr) and vanadium (V) are added     together, -   (3) the ferrite area ratio is controlled to be 10% or less, and -   (4) the bainite based metal structure is provided, in which the     carbon equivalent Ceq. (%) defined by the following equation     satisfies a range from 0.45 to 0.85, as a component composition     range to implement the high strength and the high toughness.     Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15

When the ferrite area ratio is not more than 10% and the Ceq. is in the range of 0.45 to 0.85, the strength is secured. A method of satisfying the above ranges is implemented by adjusting various kinds of contents of alloy elements and adjusting the temperature of finish rolling in the manufacturing process of the pipe. Above all, the inventor of the present invention found that when Mn, Cr and V were added together at the same time, the strength and the toughness were preferably provided in a balanced manner. In other words, when the carbon content is the same, as compared with a case the intended carbon equivalent Ceq. is attained by adding Mn and Cr only, the more preferable toughness can be provided in a case V is added while the amount of Mn and Cr is reduced to attain the intended carbon equivalent Ceq. Therefore, regarding Mn, Cr and V, it is important to add them together.

In view of the above points, the non-heat treated seamless steel pipe of the present invention comprises, by weight, C: 0.10 to 0.25%, Si: 0.05 to 1.00%, Mn: 0.50 to 2.50%, P: not more than 0.03%, S: not more than 0.05%, Cr: 0.40 to 1.50%, Mo: 0.05 to 1.50%, V: 0.02 to 0.30%, Al: 0.003 to 0.10%, B: 0.0003 to 0.01%, and N: 0.001 to 0.02%, in which the balance comprises Fe and impurities, the ferrite area ratio is not more than 10%, and the carbon equivalent Ceq. (%) defined by the following equation is 0.45 to 0.85%. Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15

According to the method of manufacturing the non-heat treated seamless steel pipe, a temperature of finish rolling at a manufacturing step is not less than 900° C. and the following relational expression is satisfied. Ceq. (%)×finish rolling temperature (° C.)≧450

In addition, a part of Fe may be replaced with one or more kinds selected from Ti: 0.005 to 0.2%, Cu: 0.05 to 1.5%, Ni: 0.05 to 1.5%, and Nb 0.005 to 0.2%.

The above specified reason will be described hereinafter.

C: 0.10 to 0.25%

C is an effective element to secure quenching ability and it is an important element to constitute the metal structure with a mixture of bainite and ferrite (pearlite) in a state after hot rolling and to provide necessary strength. In this respect, although it is necessary to contain carbon 0.10% or more, since the toughness and weldability are lowered if the carbon is too much, an upper limit value of the carbon content is set at 0.25%. In order to provide the most preferable balance between the strength and the toughness, the content of C is preferably in the range of 0.15 to 0.23%.

Si: 0.05 to 1.00%

Si serves as a deoxidizer at the time of melting. When Si content is less than 0.05%, deoxidization is not sufficiently implemented, so that non-metallic inclusions are increased. However, when the Si content is too much, the toughness is lowered, in addition, scale formation is increased and a surface property deteriorates at the time of heating the steel material. Thus, an upper limit value of the Si content is set at 1.00%. The most preferable Si content is in a range of 0.15 to 0.40%.

Mn: 0.50 to 2.50%

Mn serves as a deoxidizer at the time of melting similar to Si. In addition, it is an element which increases the strength without lowering the toughness by being added together with Cr and V to the steel in which the content of C is lowered. In order to secure the predetermined strength, the content has to be 0.50% or more. Meanwhile, when it exceeds 2.50%, the weldability and the toughness are lowered. Therefore, the content of Mn is limited to the range of 0.50% to 2.50%. In view of securing the most preferable balance between the strength and the toughness, the content of Mn is preferably in a range of 0.16 to 1.50%.

P: not more than 0.03%

P is an impurity element which incrassates in the vicinity of final coagulating point at the time of coagulation and segregates at a grain boundary to lower the hot workability and the toughness. Therefore, it is preferably lowered as much as possible. However, since it is permissible until 0.03%, the content of P is determined to be 0.03% or less. However, in order to secure the higher toughness, it is preferably 0.02% or less and it is more preferably 0.01% or less.

S: not more than 0.05%

S is also an impurity element like P, which segregates at a grain boundary at the time of coagulation to lower the hot workability and the toughness. Therefore, it is preferably lowered as much as possible. However, since it is permissible until 0.05%, the content of S is determined to be 0.05% or less. However, the machinability is lowered in some cases when it is lowered too much. Therefore, a lower limit value of S is preferably set at 0.01% when the machinability is emphasized. Meanwhile, when the toughness is emphasized more than the machinability, the content of S is preferably 0.02% or less and it is more preferably 0.018% or less.

Cr: 0.40 to 1.50%

Cr is an effective element in improving the quenching ability, and it increases the strength without lowering the toughness by being added together with Cr and V to the steel in which the content of C is lowered. In order to secure the predetermined strength, the content has to be 0.40% or more. Meanwhile, when it exceeds 1.5%, the weldability and the toughness are lowered. Therefore, the content of Cr is limited to the range of 0.40% to 1.50%. In view of securing the most preferable balance between the strength and the toughness, the content of Cr is preferably in a range of 0.40 to 1.2%.

Mo: 0.05 to 1.50%

Mo is an effective element to improve the quenching ability and also it is an important element to secure necessary strength. In this respect, it is necessary to contain Mo 0.05% or more, but if it is applied too much, the strength is increased too much and the toughness is lowered. In this respect, its upper limit value is set at 1.50%. A preferable content of Mo is in a range of 0.10 to 1.00%.

V: 0.02 to 0.30%

V is an effective element to form carbide or nitride to miniaturize austenitic crystal grains. In addition, when it is added together with Mn and Cr, the toughness can be prevented from being lowered while high strength is maintained. In order to obtain this effect, V has to be added 0.02% or more. Meanwhile, when it exceeds 0.3%, the toughness is lowered. Therefore, a content of V is limited to the range of 0.02 to 0.30%. In view of securing the most preferable balance between the strength and the toughness, the content of V is preferably in a range of 0.03 to 0.15%.

Al: 0.003 to 0.10%

Al is an element which serves as a deoxidizer. In order to obtain this effect, a content has to be 0.003% or more. When it exceeds 0.10%, aluminum inclusions are increased and a surface defect could frequently occur. Therefore, the content of Al is limited to the range of 0.003 to 0.10%. In addition, in order to secure a stable surface quality, it is preferably in a range of 0.003 to 0.05%.

B: 0.0003 to 0.01%

B is an effective element to provide the quenching ability and it is also an important element to constitute the metal structure with the mixture of bainite and ferrite (pearlite) in a state after the hot rolling and to provide the necessary strength. In order to provide this effect, a content of B has to be 0.0003% or more, however, if it is added too much, the toughness is lowered. Thus, an upper limit value of the content is set at 0.01%.

N: 0.001 to 0.02%

N is an element which miniaturizes crystal grains together with Al and Ti to improve the toughness. However, when it is less than 0.001%, the effect is small. Meanwhile, when it exceeds 0.02%, the toughness is lowered to the contrary. Therefore, a content of N is limited to the range of 0.001 to 0.02%.

One or more kinds selected from Ti: 0.005 to 0.2%, Cu: 0.05 to 1.5%, Ni: 0.05 to 1.5%, and Nb: 0.005 to 0.2%.

Both Ti and Nb are elements which form the carbide to fine the structure to improve the toughness and segregate in a base to increase the strength. The effect can be obtained when either of them is added 0.005% or more. Meanwhile, when they are added more than 0.2%, the toughness is lowered. Therefore, both of them is limited to the range of 0.005 to 0.2%.

Both Cu and Ni are elements which improve the quenching ability to increase the strength of the steel. The effect can be provided when they are added 0.05% or more. Meanwhile, when they are added beyond 1.5%, Ni causes a strength increasing effect to be saturated and increases the cost and Cu lowers the hot workability. Therefore, upper limits of Ni and Cu are set at 1.5%.

The balance: Fe and impurities.

Ca, Mg and REM (rare-earth metal) in which 0.01% is set as their upper limit values can be contained in the impurities. Although these elements do not largely affect the strength, the toughness and the weldability, since they prevent a nozzle of a tundish from clogging at the time of casting a round billet especially, they are added in some cases. When a content of each element exceeds 0.01%, the surface property deteriorates and yield is lowered. Therefore, they may be added as impurities by setting 0.01% as their upper limits.

Ferrite area ratio≦10%

The ferrite area ratio measured as an area ratio of a ferrite crystal existing in the steel pipe to its visual field, by the means such as the optical microscope has to be 10% or less for the following reason. That is, soft ferrite grains lowers the material strength when its area ratio is increased. Since the predetermined strength is not provided if the ferrite area ratio becomes a certain value or more, it is set at 10% or less.

Carbon equivalent Ceq. (%): 0.45 to 0.85% Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15

The chemical symbols on the right-hand side show contents of alloy elements, which are defined by weight %, respectively. In addition to the restrictions of the above component composition and the ferrite area ratio, in view of preferably keeping the strength, the toughness and the weldability, it is preferable that the Ceq. is limited to a range of 0.45 to 0.85%. When the Ceq. is lower than 0.45%, the strength cannot be secured and when the Ceq. is more than 0.85%, the toughness is lowered and the weld crack is likely to be generated. In addition, in view of the balance between the strength and the toughness, it is preferably in a range of 0.46 to 0.85% and more preferably, it is in a range of 0.46 to 0.70%. In addition, when the carbon equivalent Ceq. is calculated, an element which is not contained in the steel may be calculated such that its element content is 0.

The seamless steel pipe according to the present invention can be manufactured as follows. That is, the steel having the above composition is melted in a steel converter, an electric furnace or a vacuum melting furnace, and solidified by a continuous casting method or an ingot casting method. Then, the solidified object becomes a steel pipe material as it is or through blooming. Then, the steel pipe material goes through a normal manufacturing process of the seamless steel pipe and it is air-cooled.

Although the cooling after the hot rolling is preferably air cooling by natural cooling, warm cooling such as air blasting (cooling with a wind shelter cover) or wind cooling (cooling with some wind) may be performed.

Characteristic point of the method of the present invention lies in the fact that the temperature of the finish rolling in the manufacturing step is not less than 900° C. and the relational expression such as Ceq. (%)×finish rolling temperature (° C.)≧450 is satisfied. In order to provide the bainite structure while the carbon content is kept low and the alloy element content to be added is kept low, the finish rolling temperature has to be not less than 900° C. in the manufacturing step. In addition, in order to provide the strength of the steel pipe at the predetermined level or more, the value of the Ceq. (%)×finish rolling temperature (° C.) has to be 450 or more.

According to the present invention, unlike the method disclosed in the Japanese Unexamined Patent Publication No. 2001-262275, since the rolling is completed without lowering the temperature once in the manufacturing step, the precipitates are fine because it does not become large by the reheating, so that both toughness and strength can be preferably provided.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment

Steel (sample numbers 1 to 30) having chemical composition shown in a table 1 was melted, then poured into an ingot and then formed into a billet by casting. The billet was heated up to 1250° C. and formed into a pipe by a Mannesmann mandrel type of mill. Thus, a seamless steel pipe having an outer diameter of 406.4 mm and a wall thickness of 12.7 mm was provided The steel pipe was rolled and then air-cooled. Then, mechanical characteristics of the steel pipe (sample numbers 1 to 30) was examined and shown in a table 2. In addition to the mechanical characteristics, a ferrite area ratio, a finish rolling temperature, a value of the finish rolling temperature×Ceq., a billet heating temperature, evaluation for each steel pipe are shown in the table 2. TABLE 1 C Si Mn P S Cr Mo V Al B N Ti Cu Ni Nb Ceq. Preferable range 0.1/ 0.05/ 0.5/ 0.4/ 0.05/ 0.02/ 0.003/ .0003/ 0.001/ 0.25 1.0 2.5 ˜0.03 ˜0.05 1.5 1.5 0.3 0.1 .01 0.02 0.005/0.2 0.05/1.5 0.05/1.5 0.005/0.2 0.45/0.85 Sample 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 1 2 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 3 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 4 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 5 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 6 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 7 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 8 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 9 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 10 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.51 11 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.024 0.51 12 0.18 0.23 0.94 0.011 0.006 0.69 0.06 0.07 0.022 0.0009 0.0048 0.15 0.51 13 0.18 0.25 0.92 0.016 0.009 0.83 0.18 0.08 0.036 0.0011 0.0077 0.50 0.58 14 0.19 0.22 0.90 0.011 0.010 0.53 0.22 0.10 0.018 0.0013 0.0055 0.015 0.51 15 0.19 0.31 0.98 0.010 0.011 0.51 0.38 0.05 0.029 0.0019 0.0039 0.009 0.13 0.10 0.024 0.56 16 0.23 0.22 0.85 0.019 0.015 0.54 0.90 0.06 0.034 0.0005 0.0033 0.05 0.05 0.68 17 0.19 0.22 0.90 0.011 0.010 0.53 0.22 0.10 0.018 0.0013 0.0055 0.010 0.11 0.52 18 0.19 0.24 0.94 0.010 0.007 0.53 0.21 0.06 0.035 0.0014 0.0038 0.08 0.022 0.51 19 *0.28 0.22 0.84 0.015 0.010 0.08 0.05 0.02 0.023 *0 0.008 0.13 0.46 20 0.15 0.38 *0.49 0.011 0.012 1.15 0.49 0.05 0.024 0.0011 0.0056 0.030 0.57 21 0.15 0.21 *0.43 0.014 0.009 1.08 0.48 0.03 0.017 0.0011 0.0094 0.08 0.55 22 0.11 0.26 *0.42 0.013 0.008 0.58 0.94 0.07 0.018 0.0015 0.0076 0.50 0.028 0.53 23 0.11 0.26 *0.42 0.010 0.014 0.69 0.94 0.09 0.021 0.0005 0.0074 0.52 24 *0.06 0.35 1.00 0.014 0.014 0.80 0.30 0.32 0.019 0.0022 0.0076 0.51 25 *0.27 0.23 0.55 0.013 0.007 0.89 0.60 0.10 0.025 0.0009 0.0045 0.68 26 0.11 0.20 *2.60 0.013 0.012 0.54 0.21 *0.01 0.026 0.0009 0.0048 0.70 27 0.20 0.26 0.94 0.011 0.012 *0.25 0.20 0.05 0.031 0.0007 0.0051 0.46 28 0.15 0.26 0.68 0.012 0.009 0.25 *1.60 0.06 0.032 0.0011 0.0053 0.65 29 0.22 0.25 0.90 0.010 0.010 0.25 0.20 *0 0.029 0.0008 0.0053 0.46 30 0.25 0.25 0.90 0.014 0.008 0.25 *0.03 0.09 0.018 0.0008 0.0042 0.47 *Out of preferable range

TABLE 2 Finish rolling Finish rolling Billet heating Tensile strength Ferrite area ratio temperature temperature × Ceq. temperature (TS) Yield stress (YS) Preferable range not more than 10% not less than 900 not less than 450 — not less than 640 not less than 490 Result Sampl ◯ 961 487 1250 718 549 ◯ 1 2 ◯ 971 492 1250 720 531 ◯ 3 ◯ 1004 509 1280 712 510 ◯ 4 ◯ 1011 512 1280 715 536 ◯ 5 ◯ 1019 516 1280 713 521 ◯ 6 ◯ 1044 529 1280 720 532 ◯ 7 ◯ 1031 522 1280 724 554 ◯ 8 ◯ 795 *403 1180 679 *470 X 9 ◯ 853 *432 1180 676 *473 X 10 ◯ 850 *431 1180 695 *479 X 11 ◯ 1023 518 1280 732 563 ◯ 12 ◯ 1050 536 1280 733 541 ◯ 13 ◯ 956 559 1250 770 565 ◯ 14 ◯ 998 509 1250 703 510 ◯ 15 ◯ 965 537 1250 695 527 ◯ 16 ◯ 920 624 1180 755 551 ◯ 17 ◯ 927 480 1180 685 501 ◯ 18 ◯ 985 504 1180 699 524 ◯ 19 X 950 *436 1250 *570 *364 X 20 ◯ 1025 584 1250 *496 *329 X 21 ◯ 980 534 1250 *503 *367 X 22 ◯ 965 513 1250 *565 *430 X 23 ◯ 1020 534 1280 *564 *440 X 24 ◯ 1034 528 1280 *638 *481 X 25 X 1035 703 1280 668 *476 X 26 ◯ 953 663 1280 651 *465 X 27 X 1026 469 1280 655 *432 X 28 X 1038 670 1280 678 *477 X 29 X 983 452 1250 *603 *415 X 30 X 963 456 1250 *586 *432 X *Out of preferable range

According to the sample numbers 1 to 30 shown in the tables 1 and 2, notable results will be described hereinafter.

(1) Sample Numbers 1 to 7, 11 to 18

-   -   The content of each component is within the preferable range.     -   The carbon equivalent (Ceq) is within the preferable range.     -   The ferrite area ratio is within the preferable range.     -   The finish rolling temperature is within the preferable range.     -   The value of the finish rolling temperature×Ceq. is within the         preferable range.

According to these samples, the tensile strength (TS) is not less than 640 Mpa and the yield stress (YS) is not less than 490 Mpa.

(2) Sample Numbers 8 to 10

-   -   The content of each component is within the preferable range.     -   The carbon equivalent (Ceq) is within the preferable range.     -   The ferrite area ratio is within the preferable range.     -   The finish rolling temperature is within the preferable range.     -   The value of the finish rolling temperature×Ceq. is below the         preferable range.

According to these samples, the yield stress (YS) is less than 490 MPa.

(3) Sample Numbers 19 to 30

-   -   The content of any of components is out of the preferable range.     -   The carbon equivalent (Ceq) is within the preferable range.     -   The ferrite area ratio is within the preferable range in each of         the sample numbers 20, 21, 22, 23, 24, and 26, and it is out of         the preferable range in each of the sample numbers 19, 25, 27,         28, 29, and 30.     -   The finish rolling temperature is within the preferable range.     -   The value of the finish rolling temperature×Ceq. is within the         preferable range except for the sample number 19.

According to all these samples, the yield stress is less than 490 Mpa, and according to the sample numbers 19 to 24, 29, and 30, the tensile strength is less than 640 MPa.

As described above, according to the present invention, there can be provided the non-heat treated seamless steel pipe which satisfies both high strength and high toughness.

Thus, the present invention can be effectively applied to the non-heat treated seamless steel pipe for machine structural use and the like. 

1. A high strength non-heat treated seamless steel pipe comprising, by weight, C: 0.10 to 0.25%, Si: 0.05 to 1.00%, Mn: 0.50 to 2.50%, P: not more than 0.03%, S: not more than 0.05%, Cr: 0.40 to 1.50%, Mo: 0.05 to 1.50%, V: 0.02 to 0.30%, Al: 0.003 to 0.10%, B: 0.0003 to 0.01%, and N: 0.001 to 0.02%, wherein the balance comprises Fe and impurities, a ferrite area ratio is not more than 10%, and a carbon equivalent Ceq. (%) defined by the following equation is 0.45 to 0.85%. Ceq.=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15
 2. The high strength non-heat treated seamless steel pipe according to claim 1, wherein a part of Fe is replaced with at least one kind selected from a group consisting of Ti: 0.005 to 0.2%, Cu: 0.05 to 1.5%, Ni: 0.05 to 1.5%, and Nb: 0.005 to 0.2%.
 3. A method of manufacturing the high strength non-heat treated seamless steel pipe according to claim 1, wherein a temperature of finish rolling at a manufacturing step is not less than 900° C. and the following relational expression is satisfied. Ceq. (%)×finish rolling temperature (° C.)≧450
 4. The method of the high strength non-heat treated seamless steel pipe according to claim 3, wherein a part of Fe is replaced with at least one kind selected from a group consisting of Ti: 0.005 to 0.2%, Cu: 0.05 to 1.5%, Ni: 0.05 to 1.5%, and Nb: 0.005 to 0.2%. 